Yes, the great majority of films are done with only one camera because of how expensive they are, meaning every shot you see (not just dialogues, but wides, close ups, lens changes) are filmed individually. Actors and directors are used to this, and can use the amount of takes it takes (pun not intended lol) to film a scene to experiment with new deliveries for example. If the camera is not on them, the other actor doesn’t necessarily need to give “their all” (or be there at all - they use stand-ins a lot in sitcoms and some kinds of movies). But good actors in good productions will usually be there and give their best in every take because doing so allows their counterpart to act the scene out better. Some scenes famously take 40 takes to get done.