A byproduct of the DVR/DVD/VOD age is the experience of rewatching a film and being startled that a star-to-be twinkled in the background, scarcely recognizable because he hadn't yet evolved his persona. Case in point: Viggo Mortensen as the Amish farmer in the shadow of Harrison Ford's boater in Witness (1987). Frisky as a puppy, Mortensen is conscious of the camera and has not yet developed the self-contained quality so striking in A Walk on the Moon, A History of Violence and Eastern Promises. Watching him in Witness is like seeing a tadpole before he becomes the Frog Prince.
On the other end of the spectrum, sometimes a performer so powerfully establishes herself in an early outing that she is already fully in possession of her persona. Though I had never seen them before, I remember being struck by the unreadable William Hurt in Altered States (1980), the self-possessed Meg Ryan as Candice Bergen's teenage daughter in Rich and Famous (1981) and the effervescent Drew Barrymore in E.T.: The Extraterrestrial (1982).
A classic example of the former phenomena is watching Blonde Venus (1932) and suddenly realizing that Marlene Dietrich's rather wooden society swain is a tightly-coiled Cary Grant before he relaxed into his looser-limbed persona. A classic example of the latter is settling into Jane Eyre (1944) before gasping that Jane's tragic, raven-haired friend at the orphanage is Elizabeth Taylor, aged 11, her Elizabethness already fully-formed.
Lately I've been surprised to see Robert Townsend playing basketball in a gym scene of Cooley High (1975) and Richard Dreyfuss as a stage manager in Valley of the Dolls (1967), looking like themselves but not yet themselves if you know what I mean. Can you think of other examples that have startled you?