We’re all told to maintain a consistent character Point of View (POV) within a scene. There's a good reason for this and, for genre fiction, the advice makes sense. The reader is involved with a particular character in his or her head. Jumping from one character’s thoughts into another’s breaks that tenuous link and reduces the reader involvement.
When first writing a scene, it’s possible that you’re not sure whose POV to use. Once it’s down on the page, then decide – usually the person who is affected the most emotionally. If you’re sticking with a single character POV throughout the novel, then there should not be an issue – but make sure no other character’s thoughts creep in!
Here’s a brief excerpt from Broken Silence, a first novel by Danielle Ramsay.
‘In fact he needed to make a call. One he didn’t want Conrad overhearing. He walked over …’
(This paragraph in his POV goes on for nine lines, then it’s followed by a new paragraph.)
‘Conrad studied Brady’s figure from the safety of his car and wondered what was going through his head… He watched as Brady took out his mobile phone, curious about who he was calling.’
Then the narrative switches back to Brady’s POV.
This is lazy writing and editing and unnecessary. All that needed to happen was something like this: ‘Brady glanced over his shoulder. Conrad was watching him. Doubtless wondering who he was calling. He swore under his breath. None of his business!’
Moral: double-check your POV stance.
If that switch is really important – divulging another character’s secrets or inner turmoil, think about making a ‘scene break’. Or convey the other character’s thoughts in dialogue and body and facial responses. Or consider using the character’s thoughts elsewhere, when it’s that character’s longer consistent POV. Usually, though, you can delete these POV switches without much loss to the narrative – and thus maintain a consistent link with the reader.