The roast chicken sandwich is the plain cold-chicken baseline, the quietest member of the roast set, and what defines it is restraint under pressure. Slices or shreds of cold roast chicken go on buttered bread with very little else, sometimes a leaf of lettuce or a grind of pepper, and the sandwich succeeds or fails on whether the chicken is good and the moisture is handled. Cold roast chicken is mild, lean, and slightly dry once it leaves the bird, with none of the running juice or crisp skin it had hot. There is nowhere to hide a tired bird or a mean scrape of butter, and that exposure is exactly what makes it the standard the bound and dressed versions are measured against.
The craft is rescuing a dry, mild meat without burying it. The chicken is taken from the breast or thigh and sliced or torn so it sits in even layers rather than sliding as a clump, and the build has to return the moisture the meat lost: butter to the edges at a minimum, often a thin smear of mayonnaise, sometimes a little of the bird's own roasting juice. Seasoning is not optional here, because cold chicken without salt and pepper reads as nothing; the seasoning is most of the flavour. The bread is soft and plain so it carries rather than competes, and a single crisp element, a leaf of lettuce or a few cucumber slices, breaks an otherwise all-soft filling on soft bread that would read as one note. Cut and pressed, it asks only to be made with care.
The variations are the rest of the chicken set, each foregrounding one addition. Bind the chicken with mayonnaise and it becomes the chicken and mayonnaise default; add crisp bacon and it leans club; bring sage-and-onion stuffing and it turns festive. Coronation chicken takes it into spiced, fruited territory, and chicken with tarragon or watercress is the herbed tea-room reading. Each deserves its own article rather than being crowded in here.