Well, we have two guitarists. My sister is the better player, the other is the better song writer. We have no snobbery over who writes what, so sometimes the rhythm guitarist will write my sisters solo. It just depends on who choses to do it.
Normally, I think the rhythm guitarist writes with a keyboard and guitar. What ever he (or any of us) comes up with gets recorded to midi on guitar pro. The file then gets sent round to the rest of the band, and we just build on it. The rhythm guitarist takes a slow perfectionist approach, getting every note to sound perfect. My sister will take a different approach, and improvise to the midi, and write her solo that way. It really depends on the song and who's writing it though. We never write as a band because we can't write under pressure. We work better when we're at home on our own with no time limitations.
If my sister hasn't learned the solo in time, she'll improvise a temporary one (which can become permanent) in band practise. Live, she also tends to add in extra slides and run downs here and there too.
I read somewhere that Slash improvises around a set template every time, so evcery time he plays a solo, it will sound similar but slightly different. I think I heard that the guy from sum 41 (not one of my favourite bands, but I'm ashamed to admit that I do have a soft spot for them) writes his note-by-note. Don't quote me on this though, I can't remember where I heard it so it may be complete rubbish.
So there you go. If you want to learn more about guitar solos, ask a guitarist. I'm just an average drummer.
Quote: "On another note though, IMO most over-rated guitarist ever would be The Edge from U2 - damn I hate that guy, gonna put a landmine under one of his effects pedals."
U2 are over-rated full stop.
IceBound Melodic rock/metal band with loads of lead guitar, clean female vocals, and more guitars! What more could you want?