This isn't specific to adult games and is more relevant to the designer, but start small. If your project lead is new at this, they may not know it, but their most important responsibility is controlling the game's scope. Scope creep kills most projects, especially for inexperienced teams. I'd estimate 90% of new game developers have their first project collapse under their uncontrollably expanding scope. Your project lead needs to look at every suggested addition to the game, budget the work involved, and figure out whether the value of the addition is worth it. If you don't have a project lead, then this responsibility is split among all members and much harder. Don't agree to any new additions to the game until everyone on the team has had a chance to calculate the work involved and approve it.
On a related note is the role of the game's designer. Fundamentally, the purpose of design is to get the most value for effort. In the case of games, this equation becomes enjoyment for man-hours of work. Ideally, this means making sure aspects of the game synergize in a way that provides more enjoyment than the sum of its parts. Keep this in mind every time a new feature or piece of content is suggested. The thing a lot of new developers don't realize is that regardless of whether team members are collecting a paycheck, you still have a fixed budget. That's the amount of time the members of your team have available to work on the project. Don't assume you can make up extra work by just trying harder. Don't add stuff for the hell of it. Every new piece of work you add costs hours that could be used elsewhere.
Some advice specific to adult games: Don't try to add fetish or sexual content that you don't find appealing just because it's popular. A lack of passion will show in your work.
Some general writing advice. Writing characters for games is similar to writing characters for other mediums, but you'll usually have less time to develop each character. This doesn't mean that you should just write shallow characters. You should know each of your characters likes, dislikes, background and quirks even you don't plan on sharing any of that with the player. Consider how each of those things will influence the character's actions, thought process, and manner of speech while delivering whatever text you need for gameplay purposes or to advance the plot. By keeping these things constantly in mind, you'll be able to gradually develop an interesting character without needing to devote time to explicit character building.
Some more specific writing advice (Your mileage may vary): If your player character is intended to be a player surrogate, rather than an established character, be careful with dialogue. It's really jarring to the player if their character says something they would never say. I personally handle this by never including any dialogue from the player. The narration just summarizes what was said. You can also handle this by having the player choose all of their own dialog (the Bioware/Telltale solution). Even if there's only two choices and they don't have any consequences, people will generally not be bothered by dialogue that they picked.
Thanks!
