Takes a lot of practice and time on the neck to have good intonation , if you don't have perfect pitch. In the lower end , with a band on top of you , you HAVE to be dead on 'cause you'll look like a Moron.....
Over 25 years that I play fretless , still working hard on it , not there yet , but I think I never will..... It's worth it , I play 95% fretless.
To expand a little.
Playing live with root notes is easy, You just have to be able to hear yourself clearly. Just like tuning by ear with your guitarist, you hear the frequency difference in waves if your not in tune with each other, right? same thing happens riding the root, you will hear the "wave" and adjust your fingering sharp or flat. Just a little smear or roll of the finger, and the sound "wave" will disappear. Your in tune!!!
Playing faster passages is even easier, the less time you have a note fretted, the less out of tune it sounds as long as you are close to the note. Unison lines with your guitarist that aren't perfect will sound like someone has a chorus going on, again as long as you are close to the note. Tune some mid and treble out of your sound until you get better at intonating the notes, last thing you want is a heavy mid growl that is out of tune.
Mwah is your friend.
Use mwah to find your intonation, sounds artistic, saves your butt. Vibratto with mwah allows you to adjust your intonation, and the listener thinks you meant to do it. Vibratto is not bending the string on a fretless, just fret the note and wiggle your finger/ hand .
Once you learn the above you will never be lost in the mix. You will have the choice to support the band or step up and rip. Attack is the only difference.
Oh, and try not to look at the fretboard if you can. Sure I know that is counter to what you know, and how you learned. But you have to learn to hear your intonation, not see it. Start by playing a tune you already know on fretted bass. You will already have the muscle memory, you just have to finetune the fingerings for intonation. Play it, listen to your sound, and adjust.
Easier than it sounds, that's how I started. I can't read music, know nothing of theory, and really have to stop and think when someone asks me to play in a certain key. If I can learn fretless, anyone can.
