The comment by @AyushBhatt on my answer to this question got me thinking: Why would Snape use a spell to penetrate Harry's mind during his Occlumency lessons, if he can do it without any incantation?
'Let's go again ... on the count of three ... one-two-three-Legilimens!'
Harry's throat went dry. He knew what Snape was going to do and he had never been able to prevent it... The bathroom seemed to shimmer before his eyes; he struggled to block out all thought, but try as he might, the Half-Blood Prince's copy of Advanced Potion-Making swam hazily to the forefront of his mind...