I didn't see the play, but I thought the film was pretty decent. I agree the acting's kinda stagey, but Kerry Washington and Steven Pasquale's performances are pretty great, and kinda carry the film.
I think the presentation's pretty good too. I like the idea of a film taking place in one room. With a play, you don't even think about it taking place on one set, but a film set in one place gives it a claustrophobic, almost purgatoryesque aesthetic, and I really like it.
The claustrophobic setting also feeds into the tension the direction is able to create, especially in the final third of the movie, when everyone's arguing and fighting, it's pretty dicey.
Spoiler wise:
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The part of me that's a writer says that the ending is about as blatant as emotional manipulation gets. However, the audience member in me says that the ending, seeing the two characters process and sob over the death of their son was absolutely devastating. It's something you don't want to even think about a parent going through, but seeing them endure it was heartbreaking.
I don't regret seeing it, and I think you should check it out if you want. Again, I didn't see the play, so I don't know how it measures up, but I liked it fine.