Your key role is not the melody!
If you’ve grown up playing classical music then you are probably used to reading the melody but in congregational music where the lead instrument is the vocal that role becomes redundant so you have to find something else to do.
Here are 5 ideas – very useful if you don’t have set arrangements.
Play in the gaps. Don’t just play random notes or stock licks. Use the melody as your platform and take rhythmic harmonic ideas from it to play good phrases.
Play chord tones. You could play the 1st (root) note or the 3rd or 5th notes of the chord being played. Kind of how a backing vocalist would use ohs and ahs. Again think rhythmically and try to make your notes melodic.
If there’s more than one instrument use the root and 5th notes in the key. These sound really stable over the whole song as long as it stays in key so its safe territory for multiple instruments.
Synchronise rhythmic notes. Long notes often sound more relaxed than short notes so decide on whole notes, crochets, quavers according to the feel of the song and all synchronize together.
Motifs – These are great at key points of the e.g between turnarounds like In Christ Alone or the rising motif over the verse in O Sacred King. Motifs need to be instantly recognizable and singable by the non musician so make them catchy.
Don’t get bored– learn percussion. Sometimes there’s just not room for orchestral instruments for the whole set. Its much better to make a short statement with your playing and leave them wanting more rather than overdoing it. If you have long periods when you don’t play, don’t get bored, learn some hand percussion instruments.
Or just sing! Either add harmony BVs or just worship!