Give each ad-lib a role
Ad-libs can answer the lead, underline a word or add energy between lines. They should contribute something the main vocal does not already provide.
Choose a few moments to echo a phrase or punctuate a line, rather than filling every gap by default.
Keep the lead intelligible
Ad-libs can compete with the main lyric, especially in a dense verse. Use placement and level to preserve the lead as the focus.
Pan or lower supporting layers where appropriate, and mute ad-libs under the most important words.
Vary delivery and texture
Different energy, pitch or distance from the microphone can distinguish an ad-lib from the lead. Variation should still fit the song’s mood.
Try a quieter response, a higher register or a short whispered layer, then keep only the takes that add character.
Edit and process with restraint
Delay, reverb and distortion can place ad-libs in their own space. Heavy effects on every layer can make the mix crowded.
Use a shared effect send for related layers and automate effects at selected phrase endings.