Sentences without topic particles?

Question

how common is it for there to be sentences like 「私、死んでるよ！」or 「これ、怖い」？ is it super informal? any guidelines on when to speak like this?

Answer 1

Omission of は/を (and sometimes に) is ubiquitous in casual conversations, including not-so-serious business conversations with your colleagues.

• これ何？
• ペン貸して。
• お昼食べました？
• どこ行く？

But if you think naturalness is important, note that something like 私 is usually also omitted along with は.

• 死んでる！ He's/She's/I'm dead! (after you found someone was dead, or after you woke up to find yourself transformed into a zombie/ghost)