Basically, a song has to sound and feel good. If a part sounds immediately good, it probably is. If it has to be forced to fit, it's probably not a good idea.
I agree... you can place your fill(s) anywhere in the song, but it has to fit the song, be part of it, both musically and emotionally
That's part of why certain pros are respected and in-demand... the parts they play are typically 'right' at the start.
So true, the likes of Vinnie Colaiuta, Simon Phillips or Steve Gadd can certainly throw fills in a song that almost spontaneously felt.
I read on Simon's FAQ web page that he plays a song only once or twice before recording it, it's all fresh and spontaneous, he also added that it's one of the reason he is so much in demand... he provides the right track with the right fills almost immediately.
You need to be a monster to do this in so many styles, these guys have played on hundreds and hundreds of records in various styles, can you imagine the kind of musical and technical knowledge and experience you need to be able to achieve this? That's why these guys are in such high demand and so respected in the music world.
In a trio like the Police, with the guitar style ala Andy Summers... that left a bit of space for Stewart to exercise some latitude with the drum figures. Andy left a lot of space. I love the trio sound. Nice and uncluttered.
Stewart Copeland managed the space around him brilliantly with taste, feel, innovation and emotions, that's also thanks to Andy Summers and Sting, both left lots of space for the drum to shine. It's also why we rate Stewart's playing so high, you hear everything he's doing.
Where the fill starts isn't all that important. However, where you end it can either make you look like the next genius master musician percussionist or a newb.
I agree, where you land your fill can make all the difference, like going across the bar and accenting the second beat of the next bar, it can be awesome.