Anybody who ever saw a live performance by Bert Lloyd will have been aware of his habit of 'grinning' while performing - not so much a reflection of his (I'm sure) sunny outlook on life but a deliberately contrived technique to produce a hard tone on some songs, contrary to the somewhat light, airy one he generally used.
The command and use of tones to handle different types of songs and to produce variation in a performance of a number of songs, was one of the major features of the work of the Critics Group.
MacColl also devised a whole scale of work on 'efforts' (delivery) based on Laben's theory of movement, which also helped to give a varied performance - a bit difficult to explain, but very easy to apply once you get the hang of it.   
His theory was that if you were put in a position of singing a number of songs one after the other, and you didn't alter either tone or effort, as so many of us never gave a thought to, the audience would simply stop following what you were doing because of the 'sameness' of the sound you produced - (their "ears would go to sleep").
He argued that people are constantly changing tone, weight, direction... in their speech, depending on the subject and the emotions those subjects evoked - why not in their singing.
Far from this being 'theatrical' as some people have suggested, it is a natural part of how we communicate.
Jim Carroll