In some cases, that might be as simple as acknowledging that something went wrong and apologising for that. The sooner any mistakes are identified and acknowledged, the more satisfied the complainant is likely to be.
If the complaint cannot be sorted out on the spot, then speed is still of the essence. The sooner that staff are asked what happened on the issues complained about, the easier it will be for them to describe what occurred.
In some cases, in particular where the complainant requests it, a meeting with the staff involved is helpful. Items for discussion should be shared in advance, and meetings followed up in writing, either with a letter summarising the outcome of the meeting or with formal, accurate and comprehensive minutes, or both. Although not always appropriate, it is worth thinking about recording complaints meetings with the participants’ permission.