Examples:

• When he said he was quitting his job, he was dead serious.
• She looked dead serious when she announced the new company policy.
• Despite his usual humor, his tone was dead serious during the meeting.
• The coach was dead serious about improving the team's performance.
• He was dead serious when he warned them about the dangers of the hike.
• Her dead serious expression made it clear she wasn't joking.
• The detective's dead serious demeanor indicated the gravity of the case.
• They were dead serious about the consequences of breaking the rules.
• She was dead serious when she said she would move to another country.
• His dead serious attitude showed his commitment to the project.
• The teacher was dead serious about the importance of completing the assignment on time.
• He was dead serious when he spoke about the need for environmental conservation.
• She looked dead serious while discussing the budget cuts.
• His dead serious warning about the storm made everyone prepare immediately.
• The doctor was dead serious when explaining the risks of the surgery.