I cast wall of force as a sphere (lets say about 1 foot in diameter) at the tarrasque my party is fighting. As it is cutting through the creature's space when it appears, I can choose which side the creature is on.
I choose the inside.
Will I get a beachball-sized tarrasque, crushed to death inside an indestructible wall? Can I use it to completely cut out a creature's heart or decapitate them?
Unfortunately, you can't catch 'em all.
While you may be able to encapsulate smaller monsters, the Targets section of the spellcasting rules (PHB p. 204, BR p. 102) state
"To target something, you must have a clear path to it, so it can't be behind total cover."
Since you target a point - you must be able to see that point, and as framed, you cannot currently see inside the Tarrasque.
But let's say you could, for whatever reason. The paragraph continues with,
"If you place an area of effect at a point that you can't see and an obstruction, such as a wall is between you and that point, the point of origin comes into being on the near side of that obstruction."
So while Wall of Force allows the caster to decide whether or not to shunt the creature forward, it ensures that the dome comes into play on the outside of the Tarrasque.
Seamus asked, "Wait... Can't you target a point near the Tarrasque that you can see, but still creates a sphere that intersects the Tarrasque?"
The rules for Area of Effect can be found in the next section of the rules. The rules state,
"A spell's effect extends in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area."
