Related Clues:

• Standing upright
• Absorbed in
• Standing at attention
• Like a soldier at attention
• Standing with proper posture
• Not at all bored
• Undivided, as attention
• On two feet
• Lost in a good book, say
• Engaged, and then some
• Not at all distracted
• All eyes and ears
• Fully engrossed
• Riveted with attention
• Glued to the tube, say