I was wondering if spells like Shield wizard spell could be used as an action instead of as a reaction in response to some trigger? Shield spell gives protection until the start of casters next turn, so would it be within the rules to cast it as an action in my turn?
If the casting time of a spell is 1 reaction, you can only cast it using your reaction (unless you benefit from a special feature that lets you do otherwise). This is because there is no general rule that lets you change the casting time of a spell from 1 reaction to 1 action.
There's nothing I can cite that confirms that because it's impossible to prove a negative. Rather, there simply isn't a general rule that would let you do what you're asking for.
However, this prohibition is heavily implied by the rules for casting times and for reaction spells (see PHB p. 202, emphasis mine):
Most spells require a single action to cast, but some spells require a bonus action, a reaction, or much more time to cast....
Some spells can be cast as reactions. These spells take a fraction of a second to bring about and are cast in response to some event. If a spell can be cast as a reaction, the spell description tells you exactly when you can do so.
So when the casting time of a spell is "1 reaction," you are required to use your reaction to cast it in response to the exact event or timing indicated in the spell's description. You can't choose to cast it in another fashion (again, unless you benefit from some special feature that lets you do so).
You cannot cast a reaction spell like Shield as an action, but you CAN cast it on your turn.
You can cause a trigger for the reaction during your turn, however, you would still need to use your reaction for the round to cast the spell.
- Punch yourself, cost 1 action. (or provoke an Attack of Opportunity or cast the Magic Missile at yourself).
- Cast shield as a reaction to being hit by an attack, cost 1 reaction.
The action economy of this is not ideal. The ONLY reason I can think to want to cast it as an action is if you plan to have your reaction free later in the round (eg. to counterspell) which is not doable (aside from somehow getting another turn during that round)
I agree with the answer by Bloodcinder that actions, and reactions and even bonus actions seem to be different kinds of things in 5e and hence no, you cannot cast reaction spells as actions.
In the original printing of the DMG, there was a way to cast reaction spells as actions: you could write them into scrolls and read them (quoting from page 200):
If the spell is on your class's spell list you can use an action to read the scroll and cast its spell without having to provide any of the spell's components.
However this was rewritten in the DMG errata to read:
Casting the spell by reading the scroll requires the spell’s normal casting time.
Finally, for the specific spell Shield, there is still a way to cast it as an action; through the use of the Staff of Defense. This was confirmed by Jeremy Crawford, the lead 5e rules designer, in a tweet:
That staff does, indeed, let you cast shield as an action. A DM could also allow casting the spell as a reaction.