Dom was taken aback by Orlando's beauty, when he first saw him. He took every opportunity after that to spend time with him, either watching him being Legolas (even when Dom didn't strictly speaking have a reason to be on set), or just watching him being himself in whatever bar or party they happened to be drinking in.
Orlando had had an enormous crush on Dom, back in the Hetty Wainthropp days when they'd both been teenagers. He still had copies of the "drag" and "shower" episodes.
Both were convinced their interest was not returned. So neither ever spoke up.