About twice a year, I find myself in a jam with a fiddler who doesn't count. It's a nuisance if I'm playing mando, and deeply challending if I play guitar.
To hear what I mean, start a metrononome in your head or tap your foot and sing along: "Yan-kee-Doo-dle-went-to-town-a-ri-ding-on-a-po-ny-stuck-a-fea-ther-in-his-cap-and-called-it-maca-ro-ni-Yan-kee-Doo-dle-keep-up-Yan-kee-..." with everything taking exactly the same amount of time.
It should be "po-oo-ny-yy" and "ro-oo-ny-yy" Those notes are each twice as long as the others -- half notes instead of quarter notes.
Frustratingly, these folks are often very good fiddlers, because they practice a huge amount. At home. By themselves. They can play very note-y versions, loaded with interesting ornamentation, blazingly fast. Just not with anyone else.
It's easy to describe this problem, and hear it in your mind's ear, on tunes that have lyrics. For strictly instrumental tunes, giving notes the wrong time values gets you all the right notes, but is a jam-buster. Other musicians can't play along with you, and dancers can't dance to you.
I ran into one young fiddler at a festival a few months ago who launched a common tune at break-neck pace. It could have been fun, but none of use could follow his rhythm. I suspect he went home telling himself that all his practicing was paying off because none of the other fiddlers could keep up with him.
Why's it always fiddlers? Because the rest of us keep time with our right hands: up-down, up-down, up-down, up-down. (Okay, mandolin players who still play notes in random directions fall prey to this, too, but they're much rarer.)
It's also a key to how to help fix the problem when I'm playing with such a fiddler. This doesn't always work, but sometimes it really does, so it's worth a shot: Play all half notes as a pair of quarter notes. In fact, play them as four eighth notes. Let every single up and down ring out. If they're listening, they'll sometimes follow you. Even if they don't have the tick-tick-tick-tick metronome going in their heads, they'll do the right thing if you put it in their ears.
It's also a nice ornament.
I've never heard anyone talk about it, but turning a long note into repeated eighth notes -- not a trill or a slide or a pull off, but just plain eighth notes -- sounds good. It's not fast enough to be a tremolo, but it's kind of an "eighth-o-lo."
Unlike a tremolo, though, only the first note is full volume. Make the extras soft enough not to be confused with melody notes, but still loud enough to hear. "a Ri-ding On-a Po (ke-tuck-a) Ny (ke-tuck-a)."
Emphasize the first, then whisper the rest.
Goin' Upstairs, which I learned from little Susie Levitas, a banjo player from Georgia, illustrates this nicely. The first part sounds like "This old man, he played one, / He played knick-knack on my thumb." It's a charming, half-sized tune, with four bars to a part instead of eight. Like Blue Goose or other half-length tunes, it won't work at a dance, but it's a rocking tune at a late-night jam. Each half-part (one, thumb) ends on a half note, which I often turn them into eighth-o-los.
Sometimes I make up words, singing anything that comes to mind. "This old man, he played nine, / He played knick-knack on my spine." Whatever.
As an aside, yes it seems like you should be able to just play half-length tunes for contras, because you just play them twice for each time through the dance. And no, it doesn't work. Pity.
