I recently found Quadrat while surfing: it is an aperiodical newsletter published by Franke & Heidecke, the company behind the historical Rolleiflex TLR. The first issue is about portrait and includes an interview with Steve Pyke (author of the Keith Richards portrait above) and one with Inta Ruka.

They deal very differently with portraits: to Pyke, portraiture is an attempt to discover, by investigation, and then show his findings: you could call it an investigative method. In his words we find an actively sought distance AND the realization of the meanings a face bears, whether you want it to or not. Ruta, born and living in Latvia, is a more empathetic photographer: she portrays people, not faces. The subject can't be separated from his story, you always get a context that helps you build a character around the person.