For a person whose gender is unknown, 'they' works. It's been used for centuries. I'd agree with Toby, that a shadow, etc, could be an 'it'. And you can of course use the impersonal, "I couldn't see who it was". But I personally don't like using 'it' about a specific person, even if one cannot tell their gender. I wouldn't normally even use 'it' about a specific animal.
Their movements could be fluid, or gentle, or fast, or very still except for their hands, or eyes. Often, it's the little things that grab me as a reader, rather than the grand gestures.
When it comes to speech, not all voices are identifiably masculine or feminine, especially if the words are spoken softly or whispered. So 'their voice', or even 'the voice' can be used in certain circumstances, but that said, speech can give indicators, from sex and gender, through accent, to health and mental state (pausing, hesitant, staccato, breathless). So, it could be a place to bring in clues - if you want to,
At the end of the day, unless you're going for a third person omniscient narration, then you're writing what the central character of the scene can discern, and how they interpret it.