I realized today that lots of people are writing their scripts with 3 acts. I'm writing a screenplay. I understand it's a basic format, but is it necessary in a screenplay? Because so far I haven't been following it. Or I don't think I have, it might be subconscious.

NaNo08 - Darkside - Won

Screnzy09 - Darkside - Won

Of course not. Three acts is a guideline to follow, but it certainly isn't necessary to follow it. Have one act if you want to. Have five acts if you want to. Have fun with it.

Three acts is the norm, but other structures are possible.

One of my favorite movies, Scarface, has two distinct acts. The first major act has Tony making his way to the top of the Miami underworld, and act two has him on top of the Miami underworld and having to deal with things going straight to hell.

Sweeney Todd, being a musical, also has two distinct acts. The first act has Sweeney plotting revenge against Judge Turpin for what happened to him and his wife, and the second act has him becoming the legendary Demon Barber of Fleet Street after his first attempt at revenge at the end of act one goes awry.

I always thought there were three acts even in said films (not that I've watched them), but it usually goes:
First act, introducing the characters and their conflict.
Second act, overcoming conflicts and things going relatively smoothly.
Third act, everything goes awry, including the "third act twist", a surprise turn in events.

In my three years of doing Screnzy!, I have never done three acts.
I've done as many scenes as I wanted and/or got me over the finish line.

OH MAA GAA NANO COOKBOOK

Any structure will fly. The important thing is that the reader can follow it. If at the end, everything in the screenplay makes sense to your reader, not you, then you're good as gold. =)