Occasionally in a script, you might want to cut back and forth between two or more scenes. These scenes are occurring at the same time. Instead of repeating the Scene Heading for each scene over and over, an INTERCUT is used. This gives the reader the sense that the scene is moving rapidly back and forth between locations. There is a great sequence of intercuts in The Deer Hunter of shots of hunters out in the woods with a wedding going on simultaneously, at a different location. Here's another example:
INT. SHERRI'S APARTMENT - NIGHT Sherri starts disrobing in front of her open bedroom window. INT. LENNY'S APARTMENT - NIGHT Lenny gets up to cross to the fridge to get a beer. He looks out his window and catches a glimpse of Sherri across the courtyard. He freezes, watching her. INTERCUT BETWEEN LENNY AND SHERRI Sherri sits on the bed and unbuttons her double-breasted suit jacket. Lenny moves closer to the window for a better vantage point. Sherri stands, hopping a few feet, trying to step out of her skirt. Lenny, eyes glued to Sherri, moves to keep her in view. He slams his bare foot into a dumbbell on the floor. LENNY Ow! Sherri hears the yelp and looks in Lenny's direction. Lenny sees Sherri and DROPS from her view.
Another type of INTERCUT is used when two characters are on the phone and you don't want one half of the conversation to be O.S. - you want to show them both.
INT. LIVING ROOM - NIGHT Sherri, comfy on the couch, is reading a book when the phone rings. She answers it. SHERRI Hello? EXT. PHONE BOOTH - REST AREA Lenny sips a Coke as he talks. LENNY Hey Honey, I'm in Barstow. INTERCUT BETWEEN LENNY and SHERRI SHERRI Oh, Honey, that's great... you'll be here by morning. LENNY Yep... I've got the pedal to the metal.
In older films it was common to use a split-screen to show such a conversation. It's not common these days, and unless you have a very good reason for writing it in, it is best to INTERCUT.