#The spellcasting is disrupted

In general terms, you have started casting a spell, but then while you were doing that something happened that prevents you from casting the spell.

So the question is this: if you are in a condition that you could not do an action, can you (just) finish/complete it? It would make no sense to answer yes. Consider the following:

Lets say you start casting and are rudely interrupted by an arrow that was readied to "when the caster starts casting" and fall unconscious from the damage. It would be unreasonable to say that you then finish the spell. You have aquired a condition that would prevent you from taking the action, so it is now disrupted: started but not completed.

While moving is not an Action, we can also look at the way the Sentinel feat works. Your attack of opportunity is triggered by someone trying/starting to move away from you. If you hit, their speed becomes 0. Should they then continue to follow their stated course? Of course not, they stop without actually moving any. The whole point of the feat is that you can pin enemies down, and it does so by afflicting the creature with a condition between it starting to do something and finishing it.

If you acquire a status that prevents you from taking an action while taking it, you fail to complete it.

The answers to this question about the stunned condition are also in line with this interpretation. (Thanks to Medix2 for the link.)

The spellcasting is disrupted

In general terms, you have started casting a spell, but then while you were doing that something happened that prevents you from casting the spell.

So the question is this: if you are in a condition that you could not do an action, can you (just) finish/complete it? It would make no sense to answer yes. Consider the following:

Lets say you start casting and are rudely interrupted by an arrow that was readied to "when the caster starts casting" and fall unconscious from the damage. It would be unreasonable to say that you then finish the spell. You have aquired a condition that would prevent you from taking the action, so it is now disrupted: started but not completed.

While moving is not an Action, we can also look at the way the Sentinel feat works. Your attack of opportunity is triggered by someone trying/starting to move away from you. If you hit, their speed becomes 0. Should they then continue to follow their stated course? Of course not, they stop without actually moving any. The whole point of the feat is that you can pin enemies down, and it does so by afflicting the creature with a condition between it starting to do something and finishing it.

If you acquire a status that prevents you from taking an action while taking it, you fail to complete it.

The answers to this question about the stunned condition are also in line with this interpretation. (Thanks to Medix2 for the link.)

#The spellcasting is disrupted

In general terms, you have started casting a spell, but then while you were doing that something happened that prevents you from casting the spell.

So the question is this: if you are in a condition that you could not do an action, can you (just) finish/complete it? It would make no sense to answer yes. Consider the following:

Lets say you start casting and are rudely interrupted by an arrow that was readied to "when the caster starts casting" and fall unconscious from the damage. It would be unreasonable to say that you then finish the spell. You have aquired a condition that would prevent you from taking the action, so it is now disrupted: started but not completed.

While moving is not an Action, we can also look at the way the Sentinel feat works. Your attak of opportunity is triggered by someone trying/starting to move away from you. If you hit, their speed becomes 0. Should they then continue to follow their stated course? Of course not, they stop without actually moving any. The whole point of the feat is that you can pin enemies down, and it does so by afflicting the creature with a condition between it starting to do something and finishing it.

If you aquire a status that prevents you from taking an action while taking it, you fail to complete it.

The answers to this question about the stunned condition are also in line with this interpretation. (Thanks to Medix2 for the link.)

#The spellcasting is disrupted

In general terms, you have started casting a spell, but then while you were doing that something happened that prevents you from casting the spell.

So the question is this: if you are in a condition that you could not do an action, can you (just) finish/complete it? It would make no sense to answer yes. Consider the following:

Lets say you start casting and are rudely interrupted by an arrow that was readied to "when the caster starts casting" and fall unconscious from the damage. It would be unreasonable to say that you then finish the spell. You have aquired a condition that would prevent you from taking the action, so it is now disrupted: started but not completed.

While moving is not an Action, we can also look at the way the Sentinel feat works. Your attack of opportunity is triggered by someone trying/starting to move away from you. If you hit, their speed becomes 0. Should they then continue to follow their stated course? Of course not, they stop without actually moving any. The whole point of the feat is that you can pin enemies down, and it does so by afflicting the creature with a condition between it starting to do something and finishing it.

If you acquire a status that prevents you from taking an action while taking it, you fail to complete it.

The answers to this question about the stunned condition are also in line with this interpretation. (Thanks to Medix2 for the link.)

#The spellcasting is disrupted

This is essentially like counterspelling yourself. You have started casting a spell, but then while you were doing that something happened that prevents you from casting the spell.

If you aquire a status that prevents you from taking an action while taking it, you fail to complete it. Lets say you are rudely interrupted by an arrow that was readied to "when the caster starts casting" and fall unconscious. You do not then finish the spell. You have aquired a condition that would prevent you from taking the action, so it is now disrupted: started but not completed.

The answers to this question about the stunned condition are also in line with this interpretation. (Thanks to Medix2 for the link.)

#The spellcasting is disrupted

In general terms, you have started casting a spell, but then while you were doing that something happened that prevents you from casting the spell.

So the question is this: if you are in a condition that you could not do an action, can you (just) finish/complete it? It would make no sense to answer yes. Consider the following:

Lets say you start casting and are rudely interrupted by an arrow that was readied to "when the caster starts casting" and fall unconscious from the damage. It would be unreasonable to say that you then finish the spell. You have aquired a condition that would prevent you from taking the action, so it is now disrupted: started but not completed.

While moving is not an Action, we can also look at the way the Sentinel feat works. Your attak of opportunity is triggered by someone trying/starting to move away from you. If you hit, their speed becomes 0. Should they then continue to follow their stated course? Of course not, they stop without actually moving any. The whole point of the feat is that you can pin enemies down, and it does so by afflicting the creature with a condition between it starting to do something and finishing it.

If you aquire a status that prevents you from taking an action while taking it, you fail to complete it.

The answers to this question about the stunned condition are also in line with this interpretation. (Thanks to Medix2 for the link.)