Regarding the different ideas about lined fretless intonation, here's a discussion with Gary Willis. He originally recommended the "just behind the 12th line" approach, but he now says that if he were starting over he would intonate it right on the line, because the behind the line approach messes with the relation of the lines to the scale length. This is from 2009, but I've heard him say it in much more recent videos as well:
Willis Takes on Your Questions : Ask Willis
Your mileage may vary ... as posted above, the "behind the 12th line" approach is definitely preferred by some. And either way, also as posted above, the lines are just a guideline anyway. Your ears are the authority. And the lines, like frets, represent the tempered scale anyway, which is a compromise designed to make all keys as close to being in tune as possible, with the result that each key is very slightly off. We don't tend to hear it, because we're so used to the tempered scale. With a fretless, you can explore non-tempered approaches such as just intonation. Of course, this also depends on who you're playing with: keyboards are usually locked into the tempered scale for example, and you need your approach to fit the ensemble. But if you're playing a solo piece, or playing with singers and a string quartet, just intonation might be appropriate. It's all context-dependent, and fretless gives you the flexibility to choose.
A few more pieces of advice to go along with the great tips already posted by others:
1. Don't do vibrato or bends across the fingerboard like you do on a fretted bass. You don't need to, because the frets are what forced that technique, plus it will tear up the fingerboard. With no frets, you do vibrato by rocking your finger back and forth along the length of the string.
2. Don't go overboard with sliding into notes just because you can. It can be very expressive and a perfect fit in spots, but it should be an ornament that you choose to add to a line, not your default way of playing.
3. This goes along with the above advice to play everything on fretless that you play on fretted: work on being able to play a fretless without anyone knowing you're playing a fretless. You should be able to go back and forth from that approach to your more melodic and expressive fretless style effortlessly, depending on what best serves the song.
Finally, best wishes! Fretless is a wonderful thing, and it's what I've always much preferred. I play it almost exclusively, and it's very rewarding.
Willis Takes on Your Questions : Ask Willis
Your mileage may vary ... as posted above, the "behind the 12th line" approach is definitely preferred by some. And either way, also as posted above, the lines are just a guideline anyway. Your ears are the authority. And the lines, like frets, represent the tempered scale anyway, which is a compromise designed to make all keys as close to being in tune as possible, with the result that each key is very slightly off. We don't tend to hear it, because we're so used to the tempered scale. With a fretless, you can explore non-tempered approaches such as just intonation. Of course, this also depends on who you're playing with: keyboards are usually locked into the tempered scale for example, and you need your approach to fit the ensemble. But if you're playing a solo piece, or playing with singers and a string quartet, just intonation might be appropriate. It's all context-dependent, and fretless gives you the flexibility to choose.
A few more pieces of advice to go along with the great tips already posted by others:
1. Don't do vibrato or bends across the fingerboard like you do on a fretted bass. You don't need to, because the frets are what forced that technique, plus it will tear up the fingerboard. With no frets, you do vibrato by rocking your finger back and forth along the length of the string.
2. Don't go overboard with sliding into notes just because you can. It can be very expressive and a perfect fit in spots, but it should be an ornament that you choose to add to a line, not your default way of playing.
3. This goes along with the above advice to play everything on fretless that you play on fretted: work on being able to play a fretless without anyone knowing you're playing a fretless. You should be able to go back and forth from that approach to your more melodic and expressive fretless style effortlessly, depending on what best serves the song.
Finally, best wishes! Fretless is a wonderful thing, and it's what I've always much preferred. I play it almost exclusively, and it's very rewarding.