They pretend for different reasons. Initially the fear the repercussions of having Parris find them taking part in witchcraft activities with Tituba in the woods. Secondly, Abigail threatens physical violence on the girls if they don’t follow her lead. She, of course, is pretending so she can condemn Elizabeth Proctor, who fired her from her position as the Proctor’s house servant for having an affair with her husband John.
The girls fear Abigail and follow her for this reason initially; however, look at the attention and power the girls obtain by being the accusers. Think about the historical context of the work. When and how would girls of this age ever have this kind of power? Some of the most prominent figures in the town are fascinated with them. I think this attention and power becomes increasingly intoxicating for the girls as the work progresses.
These girls remind me of teens today who might find a ouija board up in the attic. They are curious and want to find out if there really is something magical out there. Tituba represented that ouija board. She being from a voodoo culture background probably had the girls curious. So, they went out to learn what they could. Most parents in any culture, but particularly in Puritan culture, would be mortified to know what these girls were getting into. Many adults do believe in a heaven and a hell, a good and an evil, a God and a devil.
The fear of getting caught made the girls fake the rest of what happened. When we lie, we often support our lies with... yep you guessed it... other lies.
I think Tituba was likely practicing what she knew to be witchcraft. I think Abigail sincerely wanted to find out more about it. The other girls were likely along for the ride. But after the original event in the beginning, they all lie and fake it in fear of being caught.
The girls are definitely just pretending. Mary Warren tells us they are and it really makes sense that they are.
It's for different reasons. Betty Parris, at the start of the play, is faking it so that she will not be in so much trouble. She knows her dad will be really angry if he finds out what she's been doing so she fakes her coma.
Then Abigail is pretending so that she can get Elizabeth Proctor in trouble. Then, I think, the girls keep pretending because they like the attention and the power.