Babies sleep patterns can deceive us.
Young babies under the age of 3 months twitch, wiggle, stretch, and thrash. They may frown or smile or spring into a burst of sucking movements. They may vocalise too. Babies in REM may create a false impression that they are awake, and the same can be said for babies in transitional sleep. During transitional sleep, newborns become more likely to vocalise, and sometimes even open their eyes. It's really only during "quiet sleep" that a young baby presents us with reliable cues that they are asleep. From the occasional sigh, their breathing becomes slow and regular, and they hardly move at all. But a young baby spends only a small quantity of their time in quiet sleep, approximately 20 minutes per sleep cycle.
To avoid this, be patient and be observant and get to know your baby's quirks before responding. Babies can awaken quickly if they hear the sound of your voice.