You would have to add more details about the question you want to address.

If you want to explore by mutagenesis the critical residues involved in a protein interaction, phage display is useful, of course. If you want to modify the interaction by in vitro evolution, its also a good choice. Its not very expensive and the most important thing, it can work in any simple molecular biology lab without the requirement for specialized equipment. The technique is quite robust.

It all depends on what you trying to find out!

For protein-protein interaction - try using Size exclusion chromatography (analytical column) to form a complex. This does not require much protein and it is done in solution.

SPR and ITC can be used to determine the kinetics/ thermodynamic of the interaction.. You'll have to buy in SPR chips and reagents etc to use on an SPR instrument - not cheap! For the ITC, you need a lot of material ( i.e. Protein or molecules you want to test for interaction) for interaction in solution.

Immuno-precipitation (need antibody) of cell lysates if you believe that interaction is being form in a cell base system..

1. Co-immunoprecipitation: Verify interactions between suspected interaction partners, not a screening approach. Reveal direct and indirect interactions, could include bridging proteins, nucleic acids (DNA or RNA), or other molecules.

2. Bimolecular fluorescence complementation (BiFC): Used to screen protein-protein interactions and their modulators.

3. Affinity electrophoresis

4. Pull-down assays

5. Yeast two-hybrid screen: Investigates the interaction between artificial fusion proteins inside the nucleus of yeast.

6. Surface plasmon resonance

7. Fluorescence resonance energy transfer: Observing the interactions of only two different proteins.

8. Bio-layer interferometry

Hope it helps!