What kind of dialogue? Branching? If so, you might want to use a branching dialogue editor such as Chat Mapper, articy:draft, or even Twine. Long ago, some writers would use Excel and specify links to different rows. It's a low tech and manual process, but everyone has access to some kind of spreadsheet software. I've also worked with some studios that have used the Aurora toolset family (available to modders in the Neverwinter Nights and Dragon Age editors), exported to XML, and then imported the XML into their own projects.

The best dialogue writer I worked with usually started with a whiteboard. At the top, she'd write down the purpose of the conversation to keep it in mind as creativity took her attention down different paths. Then she'd brainstorm different topics and goals for the conversation in a list on the side. After that, she'd write each topic on a post-it note and structure them in a dialogue tree, drawing arrows between them on the whiteboard. As she expanded each topic, she'd replace the post-it note with a large boxed area on the whiteboard containing more-detailed post-its. The process was similar to mind-mapping.

After snapping some pictures in case someone accidentally erased the board, she'd usually replicate the same thing by hand over multiple pages in a notebook, filling out the the full lines of dialogue, and then transfer them to whatever writing software the project was using, such as Chat Mapper. Chat Mapper and articy:draft are nice because they have simulators in which you can playtest your conversations, and they can export different formats including screenplays for producers and voice directors.

What you don't want to do is sit down at a blank screen in your editor and start writing your branching dialogue without a plan. It's fine to write stream of consciousness to explore different ideas as prep work, but branching dialogue needs to be structured to make sure the links all go to the right places and that you properly cover all the required topics.

On 1/6/2018 at 11:15 AM, blenderpunch said:

Do you tend to write it like a movie script? Or is there a specific format I should be following? Or a recommended one?

I assume it's branching dialogue.  Think of it like a "create your own adventure" book. Write the dialogue, and give each discrete segment (an onscreen story segment, or a spoken line of dialogue that is displayed and/or heard at one specific moment in the game) a name with a number in it (perhaps the speaking character's initials and a number). At the end of each discrete segment, a non-displayable or non-spoken instruction for the programmer as to where to jump to next (what are the next possible lines of dialogue, or the next possible scenes). 

I would recommend aristeer for this purpose.  It is a great piece of software designer for this exact purpose.