Dialogue Split by Action

When a character is speaking and then there's some Action and then the character continues speaking, you indicate the continuation like this:

                    DR. STRANGE
          This will be my crowning achievement!
          Life from life. Reincarnation while
          in the body.

Zelda enters.

                    DR. STRANGE
               (continuing)
          Never again will death be feared. I
          have bested the universe.

Zelda slowly reaches for the power cord to Strange's 
machines.

                    DR. STRANGE
               (continuing; sobbing)
          Why couldn't I have been just one
          month faster, poor Eliza, my love?

Notice that if you have a continuing indicator and a Parenthetical, you put the continuing first, then add a semi-colon and a space, then type the Parenthetical.

In TV and Sitcom scripts, the continuing indicator is placed as an Extension and abbreviated CONT'D, like this:

                    DR. STRANGE
          This will be my crowning achievement!
          Life from life. Reincarnation while
          in the body.

Zelda enters.

                    DR. STRANGE (CONT'D)
          Never again will death be feared. I
          have bested the universe.

Some people don't use continuing indicators. If you know you shouldn't use them, don't, otherwise you'll be fine if you do.

With Scriptware

If you're using Scriptware, don't worry about typing "continuings." Scriptware puts them in (or takes them out) for you, as you write!