The basics of how to play ...
Harp rests against the right shoulder, with the right hand on the upper ranges for melody and the left hand on the lower ranges for accompaniment. Thumb and first three fingers are used – no pinky. Elbows are out and raised – not parallel to the floor like a chicken, but elevated to aid in maintaining proper hand position. Thumbs up, fingers down, and palm facing the strings. A properly played note brings the finger straight back to connect with the palm.
Nylon or gut-strung harps are played with the pads of the fingers; wire harps are played with the fingernail. (Difference #2!) Corrolary to this, nails make gut/nylon-strung more difficult – they can even catch on adjacent strings, if they’re long enough. So if your harper is playing a gut-strung harp, they’re not going to have pretty, sculpted nails.
Instrument range: the usual base range for a floor harp is somewhere between 1.5 to 2 octaves below middle C. Some will run down to the A below that. The upper range is typically 3 octaves up from middle C – again with some variation. The usual base range for a lap harp is somewhere between 1 to 1.5 (ie, stopping at G or F) octaves below middle C. The upper range is typically 2.5 to 3 octaves up from middle C. You’re looking at floor harps usually maxing out at 36 strings. Lap harps are rarely less than 22, though I’ve seen a few, and that 22 – 26 string range is the sweet spot for a nice, portable lap harp.
Okay … break from all the numbers. Why this is important is the range for accompaniment on a lap harp is very small, and it can even become cramped on a floor harp, depending on the range of the melody. So single note or simple chord accompaniments are common. Disclaimer: musician paragraph next.
Due to the resonance of the harp, it is easy for accompaniment to sound muddy. Thus, most chord patterns use the 1st and the 5th note, optionally with the octave. The 3rd is usually eliminated in the accompaniment. When it will be included is in inversions, or with another note omitted just to create a different sound – but it would be rare to hear all three notes together because of the sustain.
It is possible to mute notes – a single note can be muted by re-placing the finger, and a range of them can be muted with the flat of the hand. You can get a jazzy, staccato sound this way (which isn’t terribly traditional, though it’s used in modern Celtic music).
Everything stated about resonance goes multifold for wire harp. Wire strings ring – and keep ringing – until muted. That means that accompaniments tend to be more sparse, and there is more emphasis on muting by re-placing the finger. I don’t play wire-strung harp, so I can’t speak further to the style, but that’s the main distinction. (Differences #3 through – take your pick.)
Harpers develop calluses, at the very least on the thumbs and index fingers, but what people don’t realize is that extended play is also rough on the shoulders. Sit there with your elbows out for a few minutes and you’ll see what I mean. Then add the weight of an instrument on one side.
That’s getting into misconceptions, the last stop …