I always struggle with POV. I have so many interesting characters I want to expose, and by just seeing them through the eyes of my protagonist, I feel like their true natures can't stand out as much as I want them to. I'm currently having this problem with two of my characters, Jarvis and Lucy. It is implied that they both have a romantic (even sexual) relationship but it's never really confirmed. The day before Lucy dies suddenly in battle, I wanted to have Jarvis and Lucy engage in a sexual scene, just the two of them, secluded from everyone, to elaborate on this relationship and how it will affect Jarvis when Lucy is gone. Problem is, I feel it's just random to slap in, yet I feel it's important for character development. Any thoughts on this? Another note: I have to switch POV at least once before this, as my main character is temporarily killed off until he makes a comeback.
If you are going with first person for the majority of the book then I think you really do need to stick with it. For what you are describing where you really badly need to show things he can't see then I think the answer is to write the whole thing in third person. You can still stay very close to your main characters perspective by using the narration to say what he's thinking, but it still gives you the latitude to move away when it's important. You can stick very tightly to that one guy, give just as clear a window into his mind. In first person switching perspective is extremely jarring IMO. For the reader who's spent the whole book getting to know this one guy suddenly being elsewhere is just so distracting. If your write the new guy similarly then it'll be hard for them to remember that it's someone different. If you write them differently then it'll feel like they just started a new book. There's ways to make it work like splitting the book into much more distinctly separate sections with different narrators but unless you are going to go to that length then I don't think you should jump out of your MCs head.
You clearly want to write this scene. Just write it. If it feels awkward you can edit it out later. Release it on the web to your fans later as a 'deleted scene' or something. I agree with LostThePlot that it could feel very jarring being the only plot change while the MC is alive, but I think it's likely that you'll get to the end of writing this scene and decide to write a bunch more Lucy/Jarvis POV scenes to insert into earlier parts of your story.
