How do you work with designers, especially when a design is hard or expensive to build?

The answer interviewers want is that you push back with information rather than either refusing or silently absorbing the cost.

• Get involved before the handoff. Reviewing designs while they are still cheap to change prevents most conflicts. Ask about empty states, long text, error states, and loading behaviour — the things designs usually omit and developers end up inventing.
• Quantify the cost. "This animation adds 40KB and drops us below our performance budget on 3G" is a conversation. "That's not possible" is a fight.
• Offer an alternative that keeps the intent. Designers care about the effect, not the implementation. Usually there is a cheaper way to get the same feeling.
• Use a shared vocabulary. A design system with agreed tokens for spacing, colour, and type removes most of the pixel-level negotiation permanently.

Note: Mentioning that you raise accessibility issues at design time — contrast ratios, focus order, touch target size — is a strong differentiator, because it is much cheaper to fix there than in code.