Do I need a heavy jacket?

Only if you plan on trekking in the northern mountains of Vietnam or Laos during December/January. Otherwise, a light hoodie for air-conditioned buses is sufficient.

Should I bring a suitcase or a backpack?

Bring a backpack. Southeast Asian sidewalks are often uneven, flooded, or nonexistent; dragging a suitcase wheel-first through gravel or puddles is a headache.

Are formal clothes necessary?

Not unless you plan on going to a high-end rooftop bar in Singapore or Bangkok. For 99% of the region, clean 'smart casual' is the standard.

Packing for Southeast Asia: A Practical Guide

Pack for hot, humid weather by prioritizing lightweight, moisture-wicking fabrics and leaving your heavy denim at home. Bring no more than 7 days of clothing, regardless of how long your trip is, and plan to use local laundry services to keep your bag under 7kg.

Choose your fabrics. Avoid heavy cottons which stay wet and trap heat. Instead, pack items made of linen, merino wool, or synthetic performance fabrics that dry within hours.
Implement the 'one week' rule. Pack enough clothes for 7 days. Even if you are gone for a month, Southeast Asian laundry services are cheap (usually $1-2 per kilogram) and have 24-hour turnaround times.
Cover up for temples. You must cover your shoulders and knees to enter most religious sites. Pack one lightweight sarong or a pair of loose-fitting 'elephant' pants that you can slip over your shorts when needed.
Use packing cubes. Use compression cubes to shrink your clothes. This forces you to pack less and keeps your bag organized, which is essential when you are living out of a hostel locker.

Do I need a heavy jacket?: Only if you plan on trekking in the northern mountains of Vietnam or Laos during December/January. Otherwise, a light hoodie for air-conditioned buses is sufficient.
Should I bring a suitcase or a backpack?: Bring a backpack. Southeast Asian sidewalks are often uneven, flooded, or nonexistent; dragging a suitcase wheel-first through gravel or puddles is a headache.
Are formal clothes necessary?: Not unless you plan on going to a high-end rooftop bar in Singapore or Bangkok. For 99% of the region, clean 'smart casual' is the standard.