The Z-values in INTERACT are based on the formula in this book:
Observing interaction: An introduction to sequential analysis
(Second edition)
Roger Bakeman - Georgia State University
John M. Gottman - University of Washington
Cambridge university press
Z-values Formula
where:
mGT = Estimate of the expected frequencies
xGT = Observed frequency value
pG+ = Sum per row / Total tallies per table
p+T = Sum per column / Total tallies per table
xG+ = Summary of the observed (=given) frequencies per given row.
x+T = Summary of the observed frequencies per target column
x++ = Total number of tallies in the table
Further details to this formula on the z-values tab can be found in chapter 7.4, formula 7.2.