In my experience, some guitar riffs can force the vocalist out of even coming up with a melody (such as Iron Man), and have to kind of sing-along with the riff instead. I find this happens a lot in many metal songs from the 80's, where guitarists were quite dominant. I've discovered this singing many cover songs from the 80's, and jamming/seeking out bands whose guitarists were so controlling I'd have to sing along with the guitar, and not above it.
I find this is much the case if a guitarist, the Yngwie/Van Halen types, write a song themselves, then thrust it upon me. There's usually not much more I can do with it at times, since it's full of riffs, hooks, fast-picking, etc, or the melody is already determined.
Now take a songwriter playing anything on a Korg keyboard, or piano, or an acoustic guitarist, and the ideas and melodies just flow... since there is so much space to do so. A musician on a Korg I was just hanging out and jamming together experimenting drove me nuts because as he switched the different settings, and playing random things almost made my mind explode with all the song ideas, lyrical passages and melodies that came out of that session. Kind of like a brainstorming session that was exhausting. A good exhausting
A great guitar riff is great, and may inspire some images in my mind or a feeling, that I can translate into lyrics and/or melody, just as long as I have relative freedom over the 'verses' to define a melody, rather than perhaps the guitar forcing me one way or another because he is playing something 'intricate' all the way through.