As with all formatting advice, the idea is to clearly express your vision without interrupting the flow of the screenplay.

One easy way to show one character interrupting another is to use double-dashes or an ellipsis to indicate the first character’s dialogue is being interrupted.

Both doble-dashes and ellipses indicate an unfinished thought, but double-dashes are usually the safer bet for interruption, as they indicate an abrupt cutoff, while an elipsis implies the idea trailed off.

Another option is to include the parenthetical “interrupting” if clarification is necessary.